1. Field of the Invention
The present invention relates to a method of manufacturing a laminated ring for use as a belt in transmitting power in a continuously variable transmission, and a heat treatment apparatus for use in such a method.
2. Description of the Related Art
Continuously variable transmissions have a power transmitting belt trained around a pair of pulleys. The power transmitting belt comprises a laminated ring composed of a plurality of rings and mounted on and held by an element of predetermined shape.
The laminated ring is straight in shape when traveling between the pulleys, and curved in shape when running along the pulleys. The laminated ring undergoes severe bending deformations due to repetitive cycles of the straight and curved states. Therefore, the laminated ring is required to have a mechanical strength large enough to withstand the severe bending deformations.
One known material capable withstanding such severe bending deformations is maraging steel. The maraging steel is a low-carbon steel containing 17 to 19% of Ni, and Co, Mo, Ti, etc. When the maraging steel is heated to a suitable temperature after being subjected to a solution treatment, it causes age hardening in a martensitic state, resulting in an ultra-high strength steel that is highly strong and highly tough. The maraging steel is highly suitable for use as the material of the laminated ring.
The laminated ring has heretofore been manufactured according to the following process: The ends of a thin sheet of maraging steel that is an ultra-high strength steel are welded to each other, producing a cylindrical drum. The cylindrical drum is subjected to a first solution treatment in order to uniformize the hardness that has been partly increased due to the heat applied when the thin sheet of maraging steel was welded. Then, the cylindrical drum is severed into rings of certain width, which are rolled to a predetermined length. The rings are then subjected to a second solution treatment in order to recrystallize the rolled structure for restoring the metal crystal grain configuration that has been deformed by the rolling process. The rings that have been subjected to the second solution treatment are corrected into predetermined circumferential lengths, and aged and nitrided for increased hardness. The rings of slightly different circumferential lengths are fitted together into a laminated ring.
In the above process of manufacturing the laminated ring, the rings are aged to cause the maraging steel to develop aged hardness thereby to impart a high strength to the rings. Specifically, the rings are placed in an aging chamber, and the interior of the aging chamber is heated to a predetermined aging temperature, e.g., 480 to 520xc2x0 C., and thereafter is held at the aging temperature for a predetermined period of time. The aging is performed in an inactive gas atmosphere such as nitrogen or the like in order to prevent an oxide layer from being formed on the a surface of the rings.
When the rings are aged, an intermetallic compound is separated out in the metal structure of the maraging steel, resulting in aged hardness for high strength and high toughness. The rings are then case-hardened because they need to have wear resistance and fatigue-resistant strength for use as the laminated ring.
The rings are case-hardened by nitriding the rings to form a nitride layer in their surface. The rings are nitrided by a gas nitriding process or a gas soft-nitriding process.
According to the gas nitriding process or the gas soft-nitriding process, nitrogen produced when ammonia is decomposed penetrates the metal structure of the maraging steel for thereby producing a nitride layer in the surface of the rings to harden the same for increased wear resistance and increased fatigue-resistant strength. If the rings are nitrided, then the rings are repeatedly heated to respective temperatures in the aging chamber and the nitriding chamber, kept at the temperatures for respective periods of time, and then cooled. Consequently, periods of time required for the aging and nitriding processes are increased, resulting in an increase in the manufacturing cost.
One solution to the above problem would be to successively age and nitride the rings in one processing chamber. Specifically, the rings are placed in the processing chamber, and the interior of the processing chamber is heated to an aging temperature and kept at the aging chamber for a predetermined period of time to age the rings. Thereafter, the interior of the processing chamber is not cooled, but the atmosphere in the processing chamber is replaced with an ammonia gas atmosphere or a mixed atmosphere of ammonia and RX gases, and the interior of the processing chamber is kept at a predetermined nitriding temperature for a predetermined period of time to effect gas nitriding or gas soft-nitriding on the rings. However, the above proposed process is disadvantageous in that the atmosphere changed for the nitriding process after the aging process tends to be not stabilized easily.
Another solution would be to simultaneously age and nitride the rings in one processing chamber. Specifically, the heating of the rings for the gas nitriding or gas soft-nitriding doubles as the heating of the rings for the aging. However, this process is problematic in that it is difficult to adjust the atmosphere in order to achieve an appropriate aging hardness and a nitride layer of a suitable depth.
It is known in the above process of manufacturing the laminated ring that the aging process causes the rings to shrink their volume thereof to reduce their circumferential lengths. It is also known that when the nitriding process is carried out, the volume of the rings is increased to increase their circumferential lengths because nitrogen penetrates the surface of the rings to form the nitride layer therein.
As a result, the rings which have been aged and nitrided suffer errors on the corrected circumferential lengths due to the reduction in the circumferential lengths in the aging process and the increase in the circumferential lengths in the nitriding process. In view of such errors, the rings are measured for their circumferential lengths after the aging process and the nitriding process, and classified according to the magnitude of errors of the measured circumferential lengths with respect to the corrected circumferential lengths.
Then, those of the classified rings which have such circumferential lengths that they can be fitted together are chosen and combined into a laminated ring. In order to allow easy selection of the classified rings which have such circumferential lengths that they can be fitted together, the errors of the circumferential lengths after the aging process and the nitriding process with respect to the corrected circumferential lengths should preferably be kept in the range of xc2x180 xcexcm.
According to the conventional manufacturing process, however, the errors of the circumferential lengths of rings after they have been aged and nitrided with respect to the corrected circumferential lengths occasionally become unduly large. After such rings are classified, they are not used and are left to stand for a long period of time until they encounter rings whose circumferential lengths are suitable for being fitted together.
It is therefore an object of the present invention to provide a method of manufacturing a laminated ring by processing rings in reduced periods of time according to an aging process and a nitriding process, the nitriding process being carried out in a stable atmosphere.
Another object of the present invention is to provide a method of manufacturing a laminated ring of excellent dimensional stability by allowing easy selection of rings which have been aged and nitrided and whose circumferential lengths are suitable for being fitted together.
Still another object of the present invention is to provide a heat treatment apparatus that is suitable for use in such a method of manufacturing a laminated ring.
According to the present invention, there is provided a method of manufacturing a laminated ring for use as a belt in transmitting power in a continuously variable transmission, comprising the steps of severing a cylindrical drum, which comprises a sheet of maraging steel with welded opposite ends, into a plurality of rings of predetermined width, rolling the rings to a predetermined length, correcting circumferential lengths of the rings, aging the rings by placing the rings into an aging chamber, heating the aging chamber to a predetermined aging temperature, and keeping the rings at the predetermined aging temperature for a predetermined period of time, thereafter, nitriding the rings by transferring the rings from the aging chamber into a nitriding chamber which is disposed independently of the aging chamber and has been heated to a predetermined nitriding temperature, while maintaining the rings at the aging temperature, keeping the rings at the nitriding temperature in an atmosphere containing at least an ammonia gas for a predetermined period of time, and cooling the rings, and stacking the rings into a laminated ring.
In the above method, the rings which have been aged in the aging chamber are not cooled, but kept at the aging temperature and transferred into the nitriding chamber. Since the nitriding chamber has already been heated to the nitriding temperature, the rings transferred into the nitriding chamber are directly kept at the nitriding temperature for the predetermined period of time and nitrided in the atmosphere containing at least the ammonia gas.
Therefore, a process of cooling the rings after they have been aged and also a process of heating the rings to the nitriding temperature to nitride the rings, which have heretofore been employed, are omitted, and the total processing time is reduced. Furthermore, since the nitriding chamber is disposed independently of the aging chamber, it is not necessary to change atmospheres between the aging and nitriding processes, and the rings can be nitrided in the stable atmosphere.
The step of nitriding the rings may comprise the step of gas-nitriding the rings by keeping the rings at the nitriding temperature in an ammonia gas atmosphere for the predetermined period of time or gas-soft-nitriding the rings by keeping the rings at the nitriding temperature in a mixed atmosphere of ammonia and RX gases for the predetermined period of time. The ammonia gas atmosphere may contain other inactive gases than pure ammonia.
In order to perform the aging process and the nitriding process smoothly, the rings should preferably be nitrided at the nitriding temperature which is equal to or higher than the aging temperature.
The method may advantageously be carried out using a heat treatment apparatus in which the aging chamber is connected to the nitriding chamber through an operable and closable door. In the heat treatment apparatus, the rings are transferred from the aging chamber into the nitriding chamber through the operable and closable door.
Inasmuch as the aging chamber is separated from the nitriding chamber by the operable and closable door, the atmosphere containing at least the ammonia gas in the nitriding chamber, i.e., the ammonia gas atmosphere or the mixed atmosphere of ammonia and RX gases can be maintained in a stable state. When the aging of the rings in the aging chamber is finished, the door is opened to allow the rings to be quickly transferred from the aging chamber into the nitriding chamber.
The method may further advantageously be carried out using a heat treatment apparatus in which the aging chamber is connected through a first operable and closable door to an intermediate chamber which has been heated to a temperature equal to the aging temperature and the nitriding temperature or a temperature intermediate between the aging temperature and the nitriding temperature, and the intermediate chamber is connected to the nitriding chamber through a second operable and closable door. In the heat treatment apparatus, the rings are transferred from the aging chamber into the nitriding chamber successively through the first openable and closable door, the intermediate chamber, and the second openable and closable door.
If the nitriding temperature is the same as the aging temperature, then the temperature in the intermediate chamber is the same as the temperature in the aging chamber and the nitriding chamber, i.e., a gas-nitriding chamber or a gas-soft-nitriding chamber. If the nitriding temperature is higher than the aging temperature, then the temperature in the intermediate chamber is intermediate between the temperature in the aging chamber and the temperature in the nitriding chamber.
When the rings are transferred from the aging chamber into the nitriding chamber, the rings are temporarily placed in the intermediate chamber, and the effect of temperature differences on the rings upon the transfer is reduced.
When the aged rings are transferred from the aging chamber into the nitriding chamber, the intermediate chamber is effective to prevent the atmosphere in the aging chamber from flowing into the nitriding chamber, and hence prevent the atmosphere in the nitriding chamber from being unstabilized.
The rings after they have been nitrided may be cooled in the nitriding chamber or outside of the nitriding chamber. If the nitrided rings are to be cooled outside of the nitriding chamber, then the nitrided rings may be transferred into a cooling chamber which is connected to the nitriding chamber through an openable and closable door, and cooled in the cooling chamber.
The independent cooling chamber permits the atmosphere in the nitriding chamber to be stabilized. By quickly transferring the nitrided rings into the cooling chamber, the efficiency with which to transfer the rings is increased for contribution to automating the heat treatment apparatus.
In the heat treatment apparatus, the openable and closable door disposed between the aging chamber and the nitriding chamber, or between the aging chamber and the intermediate chamber, or between the intermediate chamber and the nitriding chamber, or between the nitriding chamber and the cooling chamber should preferably be vertically movable. The door which is vertically movable to be opened and closed is effective to prevent the atmospheres in the aging chamber and the nitriding chamber from becoming unstable.
When the rings are aged, they produce aged hardness for high mechanical strength. When the rings are nitrided after they have been aged, the aging may be accelerated due to the heating in the nitriding process, tending to reduce the mechanical strength which has been achieved. In the method according to the present invention, the rings are aged in a range in which aged hardness thereof is less than a maximum value, and the aged hardness of the rings reaches the maximum value when the rings are nitrided. In the description, the aging in the range in which aged hardness is less than a maximum value is referred to as xe2x80x9csub-agingxe2x80x9d, and the aging accelerated after the aged hardness has reached the maximum value thereby to reduce the mechanical strength is referred to as xe2x80x9cexcessive agingxe2x80x9d.
In the method according to the present invention, the rings are aged for sub-aging in the aging chamber, and the aged hardness of the rings is caused to reach the maximum value by the heat produced in the subsequent nitriding process, so that the aged hardness will have an appropriate level.
In order to sub-age the rings in the aging chamber, the rings are preferably aged in a temperature range from 480 to 500xc2x0 C. for less than 60 minutes. If the aging temperature were less than 480xc2x0 C., then the aged hardness would not reach the maximum value even when heated for 60 minutes, but in the sub-aged region, and a long period of time would be required to achieve the appropriate level of aged-hardness. If the aging temperature is 500xc2x0 C., when the rings are aged for a period of time in excess of 60 minutes, the rings suffer excessive aging, and the aged hardness starts to decrease. If the aging temperature exceeded 500xc2x0 C., then the rings would suffer excessive aging even when the rings are aged for a period of time less than 60 minutes.
In order to maximize the aged hardness in the nitriding process which follows the aging process, the rings are preferably nitrided in a temperature range from 480 to 520xc2x0 C. at the nitriding temperature which is the same as or hither than the aging temperature for a period of time ranging from 45 to 60 minutes. In the nitriding process which follows the aging process, nitrogen penetrates the surface of each of the rings to form a nitride layer therein to produce hardness. Each of the nitrided rings develops such a hardness gradient that the hardness progressively decreases along the depth to which nitrogen goes into the ring from the surface thereof. Since the rings are used in a stacked state, each of the rings is required to have an appropriate hardness gradient from the surface of the ring into the ring for the resistance to fatigue due to relative slippage caused between the surfaces of the rings.
If the nitriding temperature were less than 480xc2x0 C. and the rings were nitrided for less than 45 minutes, then the produced hardness gradient would not reach a required level of hardness. If the nitriding temperature were higher than 520xc2x0 C. and the rings were nitrided for more than 60 minutes, then-the produced nitride layer would be deep and the internal hardness would be lowered.
The circumferential lengths of the rings are reduced by the aging process, and increased by the nitriding process. Therefore, the corrected circumferential lengths suffer errors. In the method according to the present invention, the rings are aged in a temperature range from 450 to 500xc2x0 C. for a period of time ranging from 2 to 3 hours, and nitrided in a temperature range from 450 to 500xc2x0 C. at the nitriding temperature which is the same as or higher than the aging temperature for a period of time ranging from 30 to 120 minutes, so that the errors of the circumferential lengths of the nitrided rings with respect to the corrected circumferential lengths of the rings fall within a predetermined error range, e.g., an error range of xc2x180 xcexcm.
As described above, the rings whose circumferential lengths have been corrected are aged in a temperature range from 450 to 500xc2x0 C. for a period of time ranging from 2 to 3 hours, and nitrided in a temperature range from 450 to 500xc2x0 C. at the nitriding temperature which is the same as or higher than the aging temperature for a period of time ranging from 30 to 120 minutes. In this manner, the shrinkage of the rings in the aging process and the expansion of the rings in the nitriding process are balanced to reduce the errors of the circumferential lengths of the aged and nitrided rings with respect to the corrected circumferential lengths. Therefore, those rings that can be fitted together can easily be selected after the aging and nitriding processes.
If the rings were kept at 450xc2x0 C. for less than 2 hours in the aging process, then no sufficient aged hardness would be achieved, and the expansion of the rings in the nitriding process would exceed the shrinkage of the rings in the aging process, increasing the errors of the circumferential lengths of the aged and nitrided rings with respect to the corrected circumferential lengths in a manner to increase the circumferential lengths. If the rings were kept at 500xc2x0 C. for more than 3 hours in the aging process, then the shrinkage of the rings in the aging process would exceed the expansion of the rings in the nitriding process, increasing the errors of the circumferential lengths of the aged and nitrided rings with respect to the corrected circumferential lengths in a manner to reduce the circumferential lengths.
In the nitriding process that follows the aging process, if the rings were kept at 450xc2x0 C. for less than 30 minutes, then no sufficient nitriding would be achieved, and the shrinkage of the rings in the aging process would exceed the expansion of the rings in the nitriding process, increasing the errors of the circumferential lengths of the aged and nitrided rings with respect to the corrected circumferential lengths in a manner to reduce the circumferential lengths. If the rings were kept at 500xc2x0 C. for more than 120 minutes, then the expansion of the rings in the nitriding process would exceed the shrinkage of the rings in the aging process, increasing the errors of the circumferential lengths of the aged and nitrided rings with respect to the corrected circumferential lengths in a manner to increase the circumferential lengths.
Because the errors of the circumferential lengths of the nitrided rings with respect to the corrected circumferential lengths of the rings fall within the error range of xc2x180 xcexcm, those rings that can be fitted together can easily be selected. If the errors were greater than the error range of xc2x180 xcexcm, then those aged and nitrided rings which can be fitted together based on the classifications according to the measured circumferential lengths are not available, and the period of time in which the existing rings are left in stock is increased.
In order to keep the errors of the circumferential lengths of the nitrided rings with respect to the corrected circumferential lengths of the rings reliably within the error range, the rings are preferably nitrided such that the thickness of a nitride layer in the surface of each of the rings is in a range from 20 to 40% of the overall thickness of the ring. If the thickness of the nitride layer in the surface of each of the rings were smaller than 20% the overall thickness of the ring, then no sufficient hardness would be reached by the nitriding process, and the shrinkage of the rings in the aging process would exceed the expansion of the rings in the nitriding process, increasing the errors of the circumferential lengths of the aged and nitrided rings with respect to the corrected circumferential lengths in a manner to reduce the circumferential lengths. If the thickness of the nitride layer in the surface of each of the rings were greater than 40% the overall thickness of the ring, then the expansion of the rings in the nitriding process would exceed the shrinkage of the rings in the aging process, increasing the errors of the circumferential lengths of the aged and nitrided rings with respect to the corrected circumferential lengths in a manner to increase the circumferential lengths.